The present invention relates to a method and apparatus for degassing coating material. In the method, coating material is fed into a vacuum container inside which are means for separating gas from the coating material.
In the processing industry, the mixing of gases, such as air, with the liquids and compositions used in a process typically causes several problems. Especially when coating paper or a similar fibrous web material, gas and gas bubbles in the coating material result in roughness on the surface of the paper in coating, and even areas where there is no coating at all. This problem is greater with some coating materials than others, but the problem is emphasized particularly with those coating materials that accumulate more gas than others. For example, coating materials containing talc typically contain large amounts of gas due to the properties of talc.
The significance of the problem is further affected by the coating method used. For example, in curtain coating, the gas content of the coating may be at most 0-0.25% by volume. Otherwise the gas bound by the coating may result in uncoated areas in the material to be coated, such as paper or board.
In multilayer curtain coating, the significance of degassing is even greater. Thus, if there are, for example, three or four coating layers, the coating used to produce each layer must be degassed as carefully as possible.
For removing the gas mixed with or dissolved in coating material have been developed vacuum deaerators, a known embodiment of which is shown in FIG. 1. The apparatus comprises a rotating drum arranged inside a vacuum container, into which drum the coating material is led, whereupon the coating material rises up the inner wall of the drum by the effect of centrifugal force and is discharged from the drum as a thin film colliding with the wall of the vacuum container.
The problem with prior art vacuum deaerators is their insufficient deaeration capacity, especially with highly viscous substances. This is due to the fact that the small air bubbles contained in highly viscous coating materials are unable, even under an extremely high vacuum, that is, low absolute pressure, to grow large enough to be broken or to be distinguished due to their specific rising rate. Attempts have been made to eliminate this problem by increasing the vacuum, but as a result, the solvent used in the coating material, for example water, vaporizes extremely readily, whereupon the quality of the coating material deteriorates, for example, as a result of an increase in the solids content of the coating material following from the vaporization of the coating material solvent. Another method used involves increasing mixing times, but in that case the operational capacity of the deaerators remains too low, which means that a greater number of deaerators must be acquired. Furthermore, if the separation capacity of known deaerators is increased by increasing the size of the apparatuses, the apparatus size will become excessively large and manufacturing costs will increase markedly.